This invention relates to graft copolymers which are effective as additives to unsaturated polyester resins.
Fiber reinforced plastics (or FRP) using unsaturated polyester resins have superior qualities not only regarding strength and resistance against heat, water and chemicals but also regarding productivity and are widely utilized for the production of bathtubs, panels for water tanks, bathroom sinks and exterior body of automobiles. This invention relates to graft copolymers which are effective as additives to such unsaturated polyester resins and in particular to such graft copolymers of a prescribed structure having a polycaprolactone chain closed by an inactive end group in its branch.
It is known that thermosetting unsaturated polyester resins shrink significantly when they are cured and hence that molded products from these resins tend to crack or warp. Thus, glass fibers which are used for the purpose of reinforcement may become visible and pin holes may be generated. The surface appearance and paintability are thereby adversely affected and there is the additional problem of rendering the dimensional stability inferior.
Prior art methods of overcoming such problems included mixing a thermoplastic resin such as polystyrene, polyvinyl acetate, polymethyl methacrylate and block copolymer of conjugate diene compound and aromatic vinyl compound to an unsaturated polyester resin as a low profile additive (29th National SAMPLE Symposium, Apr. 3-5, 1984). Among thermo-plastic resins, however, there has been none which is totally satisfactory from all points of view including compatibility and dispersibility when mixed with an unsaturated polyester resin, ability to reduce shrinkage, surface appearance and paintability of molded products. At the present time, therefore, choices are being made only to take advantage of no more than one of the desirable characteristics.
In order to improve the situation described above, polystyrene-polyester graft copolymers having a polyester chain in the branch have been considered recently (Japanese Patent Publication Tokkai 60-99158). As methods of obtaining a polyester chain in the branch, poly-condensation of dibasic acid and glycol, ring-opening polymerization of polycaprolactone and ring-opening polymerization of acid anhydride and alkylene oxide have been disclosed. The two end groups of such polyester chains are always a hydroxyl group or a carboxylic group and macromonomers are obtained by introducing a vinyl group into one of these end groups through a reaction with various reaction reagent. Graft copolymers are obtained through copolymerization of such macromonomers with other vinyl monomers.
A prior art method of this kind has the following problems. Firstly, since use is made of a macromonomer having a hydroxyl group and a carboxylic group as its end groups, gel substances of unknown structure are easily produced and it is therefore difficult to produce a graft copolymer of a prescribed structure.
Moreover, if macromonomers are produced by the disclosed method, in particular, bifunctional macromonomers having vinyl groups introduced at both ends are inevitably produced as by-products and, since it is extremely difficult to remove these bifunctional macromonomers by purification, these bifunctional monomers eventually participate in the copolymerization reaction, producing a significant amount of three-dimensional gel substances.
Graft copolymers obtained by copolymerization of ethylenic unsaturated monomer onto the principal chain of polycaprolactone (U.S. Pat. No. 3,760,034) and graft copolymers obtained by copolymerization of polyaduct (such as polyamide and polyether) or polycondensate (such as polyester) onto the principal chain of polydiene-type elastomer (U.S. Pat. No. 4,670,485) have also been considered. According to these references, graft copolymers are obtained by reacting an ethylenic unsaturated monomer or a macromonomer with a polymer on the principal chain of polycaprolactone or polydiene-type elastomer in the presence of an initiator agent for radical polymerization. By these prior art methods, however, the degree of grafting of ethylenic unsaturated monomers and macromonomers are poor and a large amount of homopolymers comes to be mixed with the product. Thus, it becomes difficult to separate the desired graft copolymer from the reaction product. Moreover, it is difficult to control the degree of grafting and the graft chain length, and there also arises the problem that generation of three-dimensional gel materials cannot be avoided.
As an additive to unsaturated polyester resins, furthermore, graft copolymers obtained by these prior art methods have serious problems in that they cannot at all provide the high levels of capability to reduce shrinkage, surface smoothness and paintability required, for example, on external panels of automobiles.